Implantable medical devices include devices such as stents, grafts, valves, filters and prosthetic implants such as hip implants used in hip replacement surgery. The devices are implanted into a patient and it is usually intended that they remain implanted in the patient permanently. The implanted device is often chosen from a standard range of available sizes and shapes based on patient specific anatomy.
A stent is a deformable tubular structure which can be inserted into a vessel or passage in the body of a patient. For example, when an artery has become narrowed or constricted, an arterial stent can be implanted in the diseased segment of the artery, in order to keep the artery open and recover normal blood flow.
For a particular patient, a stent will typically be selected from an available range. Various imaging techniques are used to diagnose the type and extent of a disease and select the most appropriate stent. For an arterial stent, the selection is based on the target vessel diameter and the distribution and extent of the disease, which can be determined using imaging techniques such as IVUS (intravascular ultrasound) and angiography. The stent is chosen such that it has a length and diameter suitable to be implanted within the artery, and such that it can be expanded to the appropriate diameter to keep the artery open.
There are several problems associated with this approach. The range of stents available for implantation may be limited to a small number of different stent designs. The suitability of the stent depends on the skill and experience of the interventionist who determines the distribution and extent of the disease and selects the stent. Further, the stents are standard, off-the-shelf, uniform designs and are known to produce sub optimal scaffolding in locations of challenging, irregular, non-homogenous disease. Failure rates of existing devices are measured in single digit percentages.
Currently, the selection of the best stent from the available range relies on the skill and experience of the interventionist. The interventionist must correctly identify the features, and determine the size of the lumen correctly. The interventionist does not take into account features such as the morphology of the disease when selecting the stent, although these features are important considerations. Incorrect selection of a stent which is not the optimal stent for a specific patient from the provided range can cause complications and even failure.
Failure or complications can also occur if a stent with the optimal configuration for the type and shape of the diseased tissue is not available. The size of the artery and the length of the diseased section vary from patient to patient. Furthermore, the plaques or lesions formed on the artery walls which cause the narrowing can be of varying size and shape. The plaques may also be formed of different types of diseased tissue. For example, parts of the diseased tissue may be softer material such as fatty or fibrous tissue, whereas in other areas calcification may occur. Calcified regions of the diseased tissue may be widespread. The degree of distribution of the calcification of the artery is a secondary consideration when selecting a stent.
The location of the diseased tissue may also present complications. For example, left main stem coronary bifurcation stents in particular have a relatively higher failure rate. This is most likely due to the differing diameters of the three vessels, and the variation in the angle between the branches. A further example of a location for which complications may occur is when a stent is implanted in order to exclude an aneurysm, where percutaneous covered stenting options are used. In this case, a stent of a standard off-the-shelf design may result in blocked side branches at the aneurysmal segment of the vessel, since the stents are covered in order to exclude the aneurysm itself. Complications arising from this include paralysis, renal impairment and mesenteric ischaemia.
International patent application number WO 2007/149428 relates to a stent customization system and method. This document includes a general description of the process of patient specific stent design and contains a general description of a process of customizing attributes of a stent based on measurements relating to a specific patient. The document describes using a measured value of a patient attribute such as vessel size to select and customize a template. However, it does not take into account the variation of the disease along the section of the artery in which the stent is to be located. The document does not disclose adjusting the design parameters of the stent at different locations along the stent according to the variation in disease characteristics.
WO 2004/026178 discloses taking MRI scans of an aorta, and using smoothing algorithms within CAD software to interpolate between successive MRI images to produce a CAD model. A physical model of the aorta is then produced and used in manufacturing processes to produce a stent. The model disclosed in WO 2004/026178 does not take into account the variation of the characteristic throughout the measured part of the anatomy.
US 2008/201007 discloses a method in which a three-dimensional solid model of the blood vessel sleeve is constructed by interpolating data that represents the blood vessel sleeve surface between curvature contours. The model disclosed in US 2008/201007 does not take into account the variation of the characteristic throughout the measured part of the anatomy.
The above has been explained specifically in relation to vascular stents; however, the present invention is not limited to such. The above described problems are also relevant to the selection and design of stents for use in other parts of the body, and to other implantable medical devices such as hip implants and heart valves. The present invention seeks to alleviate one or more of the above problems.